1. Field of the Invention
The present invention relates generally to optical focus systems and more particularly to an optical system which uses a toroidal lens.
2. Description of the Prior Art
Optical data storage systems provide a means for storing great quantities of data on a disk storage medium. The data is recorded onto the disk by means of focussing a high power laser beam to create spots on the disk medium. The data is then accessed by focussing a low power laser beam onto the recorded spots of the disk and then detecting the information encoded in the reflected light beam.
Objective lenses with relatively large numerical apertures (NA), typically 0.55, are used to focus the laser beam to the spot on the medium. If a large NA lens is used, the spot size can be made smaller and more data may be recorded in a given disk space. However, the higher NA lenses also reduce the depth of focus. The focal point of a lens is actually a substantially cylindrical volume. The depth of focus is the length of the cylinder. If the depth of focus is very small, then the lens must be precisely positioned in order to keep the beam properly focussed on the medium. This places high performance requirements on the focus actuator of the optical data storage system. Therefore, it is desirable to find some way to decrease the spot size while at the same time maintaining or increasing the depth of focus.
Various references have proposed ways to reduce the focus spot size. These references include beam obscuration devices, (JP 02-206036; JP 02-206035; JP 02-91829); diffraction gratings (U.S. Pat. No. 5,132,843; JP 03-254433); phase shifting devices (Hideo Ando, "Phase-Shifting Apodizer Of Three or More Portions", Japan Journal of Applied Physics, Volume 31, 1992, Part 1, Number 2B); axicon lenses (G. Indebetouw, "Nondiffracting Optical Fields", Journal of the Optical Society of America, Volume 6, Number 1, January 1989, Page 150; JP 02-294948; JP 63-208821); Rhomboid Prisms (Yamanaka, et al, "High Density Optical Recording by Superresolution", Japan Journal of Applied Physics, Volume 28, 1989, Page 197; JP 60-191689); and other beam manipulation systems (JP 03-260602; U.S. Pat. Nos. 4,429,957; 4,790,612). However, these systems require complex optical elements and precise alignment along the central axis of a light beam. Some of the systems are also very wasteful of light. A combination of these problems makes these schemes impractical for use in optical data storage systems.
What is needed is an optical system which provides decreased focus spot size without decreasing the depth of focus and is suitable for use in optical data storage systems.